1. Field of the Invention
This invention relates to an adhesive composition, and more specifically, to an adhesive composition which, when used in adhesive films such as adhesive tapes or adhesive sheets for application on plastics, because of interaction between the carboxyl group and the amino group contained in its components, is able to impart excellent adhesion and anti-blistering characteristics to plastics under normal as well as high temperatures, and which moreover possesses excellent compatibility and transparency in itself; and also to plastic adhesive films utilizing such an adhesive composition.
2. Description of the Background Art
It has been common practice in recent years to apply an adhesive film such as an adhesive tape or sheet on the surface of transparent plastics such as polycarbonate (PC) or polymethyl methacrylate (PMMA) for the purpose of protecting such plastics. The adhesive used in such a film being applied on transparent plastics, is required to possess weathering resistance, transparency, ease in controlling adhesive power, and the like. Also, since foaming or blistering are seriously detrimental to the appearance of an adhesive film, the capability of preventing these phenomena, i.e. anti-blistering performance, is required as well.
It has been conventionally known that graft polymers, prepared by copolymerization of a major proportion of a (meth)acrylic ester and a macro monomer having copolymerizable functional groups, can be used for adhesives that can satisfy the said requirements to improve the anti-blistering characteristics for plastics.
Alternatively, an adhesive prepared by blending a main polymer containing a functional group and a low molecular weight polymer containing a different functional group that can be crosslinked with the former, and by subsequently crosslinking both polymers with a crosslinking agent is known to possess better transparency as well as excellent adhesive characteristics.
However, the above products have several shortcomings that require solutions. Namely, in the case of the adhesive mentioned earlier, impurities that have vinyl groups at both ends tend to remain in the macro monomer, making the product prone to gelation. There is also a limit in dosage from the copolymerizability as well as transparency (compatibility) considerations.
While the latter type adhesive has no significant technical problems as an adhesive, its anti-blistering performance has remained less than perfect.
Therefore, there has been a desire for the development of a technology which can economically manufactures an adhesive that possesses qualities such as weathering resistance, transparency, ease in controlling adhesive power, and anti-blistering characteristics.
The present inventor, after arduous research efforts made in an attempt to develop an adhesive that can be used for transparent adhesive films without presenting any problems, has discovered that, by combining a main polymer containing a carboxyl group and a low molecular weight polymer containing an amino group, the anti-blistering characteristics can be improved through interaction between the carboxyl group and the amino group. The present inventor has also discovered that such products will create no adverse effect in qualities such as weathering resistance, transparency, ease in controlling adhesive power, and the like. These findings have led to the completion of the present invention.
Accordingly, an object of the present invention is to provide an adhesive composition comprising (1) a carboxyl group-containing adhesive component having a weight average molecular weight of 800,000 or greater and (2) an amino group-containing adhesion-improving component having a weight average molecular weight of 100,000 or less, the adhesive composition being prepared by adding 1-40 parts by weight of the adhesion-improving component (2) to 100 parts by weight of the adhesive component (1) to crosslink the adhesive component (1).
Other and further objects, features and advantages of the present invention will appear more fully from the following description.
As an example of the adhesive component (1) containing a carboxyl group and having a weight average molecular weight of 800,000 or greater (hereinafter referred to as the xe2x80x9ccarboxyl group-containing resinous composition (1)xe2x80x9d, for simplicity) employed in this invention, a resinous composition having a weight average molecular weight of 800,000 or greater prepared by copolymerization of a major proportion of a C1-12 alkyl (meth)acrylate and 0.5-10% by weight of a carboxyl group-containing unsaturated monomer which is copolymerizable with the alkyl (meth)acrylate, can be given. Such a resinous composition can be manufactured by polymerizing (a) and (b) described below:
(a) A C1-12 alkyl (meth)acrylate: 90-99.5% by weight
(b) A carboxyl group-containing unsaturated monomer which is copolymerizable with (a): 0.5-10% by weight
Of the above, the component (a) is required to provide the product with the basic adhesiveness. Further, the component (b) is required to introduce the carboxyl group into the adhesive composition. Examples of the component (b) include acrylic acid, methacrylic acid, maleic acid, crotonic acid, xcex2-carboxyethyl acrylate, and the like. If necessary, in addition to the components (a) and (b), 0.5-10% by weight of another unsaturated monomer that is copolymerizable with the alkyl (meth)acrylate (a) may be added.
Meanwhile, as an example of the adhesion-improving component (2) containing amino group and having a weight average molecular weight of 100,000 or less (hereinafter referred to as the xe2x80x9camino group-containing resinous composition (2)xe2x80x9d), a resinous composition having a weight average molecular weight of 100,000 or less, prepared by copolymerization of a major proportion of a C1-20 alkyl methacrylate or a C3-20 cycloalkyl methacrylate having a glass transition temperature (hereinafter referred to as xe2x80x9cTgxe2x80x9d) of 40xc2x0 C. or higher and 0.5-10% by weight of an amino group-containing unsaturated monomer which is copolymerizable with the alkyl methacrylate or cycloalkyl methacrylate can be mentioned. This product can be prepared by polymerizing (c) and (d) described below:
(c) A C1-20 alkyl methacrylate or a C3-20 cycloalkyl methacrylate having a Tg of 40xc2x0 C. or higher: 90-99.5% by weight
(d) An amino group-containing unsaturated monomer copolymerizable with (c):0.5-10% by weight
Of the above, the component (c) is required to improve the adhesion to plastics. Further, as the component (d), amino group-containing compounds such as aminoethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, dimethylaminopropyl (meth)acrylate, or vinylpyridine can be mentioned.
Each of the above carboxyl group-containing resinous composition (1) and amino group-containing resinous composition (2) may be produced preferably through radical polymerization, using an organic peroxide such as benzoyl peroxide or lauryl peroxide and an azo-compound polymerization initiator such as azo(bis)isobutylonitrile, respectively, for which a solution polymerization technique in particular is desirable.
To obtain an adhesive composition related to the present invention, it is essential to blend the carboxyl group-containing resinous composition (1) and the amino group-containing resinous composition (2) mentioned above, and then to crosslink them. While the required blending ratio generally is 1-40 parts by weight of the amino group-containing resinous composition (2) to 100 parts by weight of the Carboxyl group-containing resinous composition (1), for applications requiring the best possible compatibility and transparency, it is preferred to use a blending ratio of 1-20 parts by weight for the amino group-containing resinous composition (2).
The crosslinking reaction, on the other hand, is provided for improving the adhesive performance and the anti-blistering performance of the product.
Such crosslinking reactions, specifically, can be performed by utilizing a polyisocyanate crosslinking, an epoxy crosslinking, a metal chelate crosslinking, or an amino resin crosslinking, among which the epoxy crosslinking is particularly effective. As the crosslinking agent for these crosslinking reactions, poly-functional isocyanate compounds such as tolylene diisocyanate and hexamethylene diisocyanate; poly-functional epoxy compounds such as ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, and tetraglycidyl xylenediamine; chelate compounds of Al, Ni, or Zn, etc.; and melamine compounds can be mentioned.
The adhesive composition of the present invention thus obtained as described in the above possesses excellent adhesive characteristics, compatibility, transparency and anti-blistering characteristics to plastics under normal as well as high temperatures, and therefore provides an excellent adhesive for plastics.
Accordingly, in addition to the ordinary use as an adhesive agent for plastics, the composition can be applied to a transparent sheet or a transparent tape, for instance, to manufacture plastic adhesive films or transparent protective films for plastics such as transparent adhesive sheets, transparent plastic protective sheets, transparent adhesive tapes, double-coated adhesive tapes, or transparent labels.
In particular, when the plastic adhesive film using an adhesive composition of the present invention as described above is applied to the surface of a plastic resin such as polycarbonate resin, acrylic resin, or styrene resin, advantages are obtained inasmuch as the application is able to provide effective protection to the surface of the plastic resin, while preventing foaming and/or blistering.
The excellent adhesive performance, compatibility, transparency, anti-blistering performance, and the like demonstrated by the adhesive composition of the present invention can be explained as an effect resulting from an interaction created between the carboxyl group of the carboxyl group-containing resinous composition (1) and the amino group of the amino group-containing resinous composition (2). In other words, it can be understood that an ionic attraction generated between the carboxyl group and the amino group is forcing these polymers to associate to improve their compatibility, and that the interaction at the same time increases the intermolecular force to enhance the cohesive force.
The second function of the invention comes from the crosslinking of the carboxyl group-containing resinous composition (1) and the amino group-containing resinous composition (2). In other words, as an effect of the crosslinking, any gas from the adhesive composition, if generated, can be contained in the material and prevented from creating bubbles, which results in its excellent anti-blistering performance.
In the following descriptions, a more detailed explanation of this invention will be presented based on numerous examples. However, the present invention is not in any way restricted whatsoever to those described in the examples. Note also that the proportional units used in the examples are according to weight, and that the weight average molecular weight is measured by means of gel permeation chromatography (GPC).